The present invention relates generally to optical communications, and more particularly, to a variable rate transmitter based on all-optical orthogonal frequency division multiplexing OFDM super channel technology.
Optical WDM network is the backbone for the global communication networks. As the network traffic demand increases and the services become more heterogeneous, the optical WDM network is facing demands for greater capacity and better flexibility.
To solve the capacity demand, advanced modulation formats (such as QPSK, 8-QAM, 16-QAM, and OFDM) and multiplexing techniques (such as polarization division multiplexing) have been proposed and demonstrated. These technologies lead to better spectral efficiency and higher data rate per optical channel. For example, the data rate (or called “line rate”) for a single optical channel (here “one optical channel” refers to the optical signal that is generated from a single optical source, most commonly a laser) in the WDM network has been increased from 10 Gb/s to 40 Gb/s to 100 Gb/s or higher in commercial products. 400 Gb/s and 1 Tb/s (i.e. 1000 Gb/s) optical channels have also been demonstrated. The high capacity optical channel generation techniques can be divided into 3 categories: The first type uses single carrier electrical time domain multiplexing (ETDM), where the time-multiplexed electrical data are modulated onto the single optical carrier, this is the dominating technique used for optical channels up to 100 Gb/s. The second type uses optical time domain multiplexing (OTDM), which applies time division multiplexing on both the electrical and optical domain, here the optical carrier remains single. The third type is multi-carrier modulation. It is also referred to as optical orthogonal frequency division multiplexing (O-OFDM) or super-channel. Here multiple subcarriers (or called “optical tones”) are firstly generated from the single optical source, each subcarrier is then modulated with different data. Since these subcarriers/tones are orthogonal to one another, they can be placed closely spectrally without causing crosstalk, therefore they can produce high spectral efficiency. Among the three, the super-channel method is considered as the most suitable solution for high capacity channel beyond 100 Gb/s because it does not suffers from the bandwidth limitation of electronic and opto-electronic devices compared to the other 2 methods.
To improve the network flexibility, the WDM network is evolving from the conventional “single line rate with fixed channel spacing” to “mixed line rate with fixed channel spacing” and eventually to “mixed line rate with flexible spacing”. The latter one is also called “flexible optical WDM (FWDM) network” or “flexible grid WDM network” or “grid-less WDM network”. In such network, optical channels have different line rates, and the spectral spacing between neighboring channels is non-uniform to provide the best spectrum utilization. The line rate of each channel and the channel spacing can also vary over time. And optical channels can be added or dropped dynamically at any node in the network. A key element in such FWDM network is variable rate transponder, which converts client data with variable rate to optical WDM signal and transmit it, and receive the WDM channel with variable data rate and converts it back to client data. Inside the variable rate transponder, a key element is the variable rate transmitter, which modulates the variable rate data to an optical signal.
For the first type of optical signal (ETDM), the methods to achieve such variable rate transmitter include: (I.1) varying the modulation format (such as use switches to turn on/off modulation stages, so that the modulation format can be changed between QPSK and 8PSK, or between 8-QAM and 16-QAM), or (I.2) changing the rate of the ETDM modulation data, or mixture of (I.1) and (I.2). There are some tradeoffs for these two techniques. For technique (I.1), varying the modulation format will lead to different OSNR penalty and transmission impairment in the optical signal, therefore the transmission reach will be affected. For technique (I.2), the opto-electronic component can only operate within certain data rate range, and its performance will vary with the data rate. The transmission reach is also varied with the modulated data rate.
To construct a variable rate transmitter on the second type of optical signal (OTDM), each symbol period is divided into different number of tributaries. This method has even more restriction in the achievable data rate range because of the difficulties to adjust the timing properly. Also, the number of tributaries in each symbol period needs to be integer, therefore the selection is fewer.
For the third type of optical signal (O-OFDM or super-channel), varying the channel rate can be done by: (III.1) varying the number of subcarriers generated from the single optical source; or (III.2) changing the modulation data rate in each subcarrier, which is achieved electronically by changing the modulation format or changing the subcarrier data rate or both; or the combination of (III.1) and (III.2). Using technique (III.2) here has the same tradeoff as in the first type of signal (ETDM). However technique (III.1) does not suffer from significant transmission range variation, since the data format and rate in each subcarrier remains the same.
Achieving variable rate transmitter in O-OFDM system by varying the subcarrier number has been proposed and demonstrated by several research groups. So far, all these implementations are: construct the maximum number of subcarriers required, then use some filtering device (such as optical filter or wavelength-selective switch(WSS)) to select the subcarriers that are needed. The remained subcarriers are then modulated with the electrical signal (carrying data) to be transmitted.
Such a method is straightforward, but it is not energy efficient. The optical power applied to unused subcarriers is wasted after these subcarriers are filtered out. Therefore less power are applied to the useful subcarriers. Less signal power leads to worse OSNR (optical signal to noise ratio) at the transmitter side. As a result, the transmission reach (distance) of the signal becomes shorter.
Accordingly, there is a need for a variable rate transmitter based on all-optical orthogonal frequency division multiplexing OFDM super channel technology.